The invention relates to an electrochemical cell comprising
a pair of electrodes each having one electrode body made of porous base material which essentially consists of carbon particles held together by a binder and has pores which permit percolation of fluid through the electrode body, and in which the electrode body is charged with a catalyst deposited on the base material,
a membrane film which is arranged between the electrodes, contacts the two electrode bodies electrochemically, acts as an electrolyte and separator of the electrochemical cell and is made of a hydrophillic, proton-conducting polymer material, an interlayer being provided between the membrane film and the base material with the catalyst deposited thereon,
means for introducing a fluid into at least one of the electrodes,
means for passing out a fluid from at least one of the electrodes, and
means for making electrical contact with the electrodes.
An electrochemical cell of the abovementioned type is known from U.S. Pat. No. 4,876,115. This known cell employs, as an electrolyte and separator, a membrane whose material is a polymer material comprising substituted poly(perfluoroalkylene), some of the substituents being terminally sulphonated. A membrane material preferred according to U.S. Pat. No. 4,876,115 is Nafion (trademark of DuPont), which is documented, inter alia, in Rompps Chemie Lexikon and is there referred to as a membrane material on the basis of poly(perfluoroalkylene)sulphonic acid. For listings of further membrane materials, reference is made in U.S. Pat. No. 4,876,115 to U.S. Pat. No. 4,337,137.
Such perfluorinated membrane materials in accordance with U.S. Pat. No. 4,876,115 exhibit, owing to their chemical composition, at temperatures up to 100.degree. C., the long-term stability required for operating the electrochemical cell, but they are very expensive. Moreover, establishing satisfactory contact between such perfluorinated membrane materials and the electrodes is very laborious and difficult.
Hitherto, no other membrane materials have been disclosed which, in conjunction with distinctly more favourable production costs than for the perfluorinated membrane materials in accordance with U.S. Pat. No. 4,876,115, exhibit the desired long-term stability for the operation of electrochemical cells.
The thermally and temporally limited stability of membrane materials, in particular of membrane materials grafted with a styrene derivative and cross-linked with divinylbenzene, has been known for a long time, for example from "Introduction to SPE Cell Technology" by A. B. La Conti in "Proceedings of Oronzio de Nora Symposium", Venice, 15th to 18th May 1979, pages 94-127.
In this context, long-term stability is defined as follows: the membrane of an electrochemical cell is described as stable if the ohmic loss in the cell caused by the membrane resistance increases by less than 100 mV in 1000 hours at a current density of 1 A/cm.sup.2.
On the other hand, the membrane materials known, for example, from U.S. Pat. Nos. 4,469,579, 4,506,035 or 4,605,685 do show the desired long-term stability, but are very expensive and therefore have not been applied on a major scale. These perfluoroalkylene polymers or perfluoroalkylene copolymers, radiation-grafted with an optionally fluorinated styrene radical and then sulphonated, would be desirable, however, as membrane materials for electrochemical cells given their mouldability, contactability and further beneficial properties.
The preparation of such polymers and their use as a membrane material for electrochemical cells is extensively discussed, for example, in "Radiation Grafted and Sulfonated (FEP-g-polystyrene)--An Alternative to Perfluorinated Membranes for PEM Fuel Cells?" by F. N. Buchi et al in "Proceedings of the 27th Intersociety Energy Conversion Engineering Conference, San Diego, Aug. 3-7, 1992, SAE Technical Paper Series 929293, pp. 3.419-3.423". F. N. Buchi et al report that the stability of their electrochemical cells at operating temperatures of 60.degree. C. managed to reach more than 300 hours, but at 80.degree. C. all the membranes studied (those of F. N. Buchi et al themselves and also those which they had obtained commercially for comparative purposes) suffered rapid degradation. Thus none of the electrochemical cells studied by F. N. Buchi et al exhibited adequate long-term stability.
An electrochemical cell of the type mentioned at the outset is known from "Chemical Abstracts" 108(6):41131k and JP-A-62-195855. The membrane film consists of ion-exchanging polystyrene material Selemion CMV (trademark of Asahi Gaishi). On each of the two surfaces of the membrane film a layer of sulphonated polystyrene is formed by grafting. The electrodes and their base material, with the catalyst deposited thereon, are impregnated with a solution of the ion-exchanging sulphonated material Nafion (trademark of DuPont). Then the electrochemical cell is assembled by the electrodes being joined to the membrane film on both sides. Thus, on both sides of the membrane film between it and the impregnated electrode, an interlayer is formed which consists of sulphonated polystyrene which, according to the documents cited, is a gel, so that the interlayer penetrates into the electrode and ensures an effective bond between the membrane film and the electrode impregnated with Nafion. This ensures that no "sulphuric acid" can leak out--in the cited documents this obviously refers to the possible break-down products of the membrane such as, for example, toluenesulphonic acid. While this known electrochemical cell does solve the problem of long-term behaviour with respect to leakage of "sulphuric acid", the problem of long-term behaviour in operation above room temperature is not addressed at all in the cited documents.
It is also known, from EP-A-0483085, to impregnate the electrodes, or their base material with the catalyst deposited thereon, with a solution of the ion-exchanging sulphonated material Nafion (trademark of DuPont). This disclosure reports solely on the short-term behaviour of the electrochemical cell at a slightly elevated temperature and solely with the expensive material Nafion, it therefore provides no contribution to the solution of the problem of long-term behaviour in operation above room temperature with a cost-effective material.
"Radiation Grafted Membranes: Some Structural Investigations in Relation to their Behavior in Ion-Exchange-Membrane Water Electrolysis Cells" by G. G. Scherer et al in Int. J. Hydrogen Energy, 17/2 (1992) pp. 115-123 discloses the use, as the material of the membrane film, of a base polymer radiation-grafted with terminally sulphonated vinyl radicals, the base polymer used being, on the one hand, a poly(tetrafluoroethylene) (PTFE), on the other hand a low-density polyethylene (LDPE). The long-term behaviour of the electrochemical cells fabricated therewith was studied only at room temperature in the case of the LDPE-based material. In the case of the PTFE-based material comprising styrene as the vinyl radical, the electrochemical cells failed at a temperature as low as 53.degree. C. While in the case of the PTFE-based material comprising trifluorostyrene as the vinyl radical the electrochemical cells did prove themselves in terms of long-term behaviour above room temperature, the material in question is expensive. Therefore this disclosure likewise provides no contribution to the solution of the problem of long-term behaviour in operation above room temperature with a cost-effective material.